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Growth of Ultrathin Samaria Films on Pt(111)

Permanent Link: http://ufdc.ufl.edu/UFE0045609/00001

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Title: Growth of Ultrathin Samaria Films on Pt(111)
Physical Description: 1 online resource (30 p.)
Language: english
Creator: Epuri, Santosh Reddy
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2013

Subjects

Subjects / Keywords: auger -- catalysis -- earth -- film -- leed -- microscopy -- oxides -- platinum -- rare -- samaria -- samarium -- scanning -- thin -- tunneling -- ultra -- vacuum
Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The growth of thin films of samaria(111) on Pt(111) has been studied using scanning tunneling microscopy(STM), low energy electron diffraction (LEED), and auger electron spectroscopy(AES). The films were grown by physical vapor deposition of samarium in a 5 X exp(-7) Torr oxygen atmosphere. Continuous samaria(111) films were obtained by post-growth annealing at 1030 K. Furthermore, heating the low coverage film in UHV causes it to partially reduce to form SmO(100). Re-oxidation of the film reversed these changes along with improved morphology and ordering of the film.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Santosh Reddy Epuri.
Thesis: Thesis (M.S.)--University of Florida, 2013.
Local: Adviser: Weaver, Jason F.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2015-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2013
System ID: UFE0045609:00001

Permanent Link: http://ufdc.ufl.edu/UFE0045609/00001

Material Information

Title: Growth of Ultrathin Samaria Films on Pt(111)
Physical Description: 1 online resource (30 p.)
Language: english
Creator: Epuri, Santosh Reddy
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2013

Subjects

Subjects / Keywords: auger -- catalysis -- earth -- film -- leed -- microscopy -- oxides -- platinum -- rare -- samaria -- samarium -- scanning -- thin -- tunneling -- ultra -- vacuum
Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The growth of thin films of samaria(111) on Pt(111) has been studied using scanning tunneling microscopy(STM), low energy electron diffraction (LEED), and auger electron spectroscopy(AES). The films were grown by physical vapor deposition of samarium in a 5 X exp(-7) Torr oxygen atmosphere. Continuous samaria(111) films were obtained by post-growth annealing at 1030 K. Furthermore, heating the low coverage film in UHV causes it to partially reduce to form SmO(100). Re-oxidation of the film reversed these changes along with improved morphology and ordering of the film.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Santosh Reddy Epuri.
Thesis: Thesis (M.S.)--University of Florida, 2013.
Local: Adviser: Weaver, Jason F.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2015-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2013
System ID: UFE0045609:00001


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1 GROWTH OF ULTRATHIN SAMARIA FILMS ON Pt (111) By SANTOSH REDDY EPURI A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR TH E DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2013

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2 2013 Santosh Reddy Epuri

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3 To my family

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4 ACKNOWLEDGMENTS my advisor, Dr. Jason Weaver for his patience and support. I would like to recognize the valuable contrib utions of Jin Hao Zhang tow ards this research work. I would like to thank Dr. Andreas Schaefer, William Cartas and other research group members for their support Helena Weaver for agreeing to be on my defense committee. I grateful ly acknowledge financial support for this project provided by the National Science Foundation(NSF), Division of Chemistry, Chemical Catalysis program through grant number 1026712.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF FIGURES ................................ ................................ ................................ .......... 6 LIST OF ABBREVIATIONS ................................ ................................ ............................. 7 ABSTRACT ................................ ................................ ................................ ..................... 8 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ ...... 9 2 CURRENT STATE OF KNOWLEDGE ................................ ................................ .... 12 Ultra Thin Ceria Film on Pt(111) ................................ ................................ ............. 12 Ultra Thin Y 2 O 3 (111) Films on Pt(111) ................................ ................................ .... 14 3 GOALS ................................ ................................ ................................ ................... 15 4 EXPERIMENTAL METHODS ................................ ................................ ................. 16 5 RESULTS ................................ ................................ ................................ ............... 18 UHV Treatment ................................ ................................ ................................ ....... 23 Reoxidation ................................ ................................ ................................ ............. 25 6 CON CLUSIONS AND FUTURE WORK ................................ ................................ 27 LIST OF REFERENCES ................................ ................................ ............................... 28 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 30

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6 LIST OF F IGURES Figure page 1 1 Rare Earth Consumption (REO) by Application. 3 ................................ ............... 11 2 1 STM images of ceria film grown on Pt(111) ................................ ....................... 13 2 2 STM images of Y 2 O 3 film grown on Pt(111). ................................ ....................... 14 5 1 Pt step heights in the presence of Sm 2 O 3 .. ................................ ......................... 18 5 2 LEED and STM images of ~0.6 ML Sm 2 O 3 film grown on Pt(111). .................... 20 5 3 LEED and STM images of ~1.5 ML Sm 2 O 3 film grown on Pt(111). .................... 21 5 4 LEED and STM images of ~2.4 ML Sm 2 O 3 film grown on Pt(111).. ................... 22 5 5 Plots of average island diameter and lattice constant ratio with increasin g coverage. ................................ ................................ ................................ ............ 22 5 6 LEED and STM images of ~4.2 ML Sm 2 O 3 film grown on Pt(111). .................... 23 5 7 LEED and STM images of ~6 ML Sm 2 O 3 film gr own on Pt(111). ....................... 23 5 8 LEED and STM images of ~1.5 ML Sm 2 O 3 film grown on Pt(111) followed by annealing in UHV. ................................ ................................ ............................... 24 5 9 LEED and STM images of ~2.4 ML Sm 2 O 3 film grown on Pt(111) followed by annealing in UHV. ................................ ................................ ............................... 25 5 10 LEED and STM images of ~ 6 ML Sm 2 O 3 film grown on Pt(111) followed by annealing in UHV. ................................ ................................ ............................... 25 5 11 LEED and STM images of Re oxidized Sm 2 O 3 film grown on Pt(111). ............... 26

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7 LIST OF ABBREVIATIONS AES Auger Electron Spectroscopy LEED Low Energy Electron Diffraction PVD P hysical Vapor Deposition REO S Rare Earth Oxides STM Scanning Tunneling Microscopy TPD Temperature Programmed Desorption UHV Ultra High Vacuum

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8 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for th e Degree of Master of Science GROWTH OF ULTRA THIN SAMARIA F ILMS ON Pt(111) By Santosh Reddy Epuri May 2013 Chair: Jason F. Weaver Major: Chemical Engineering The growth of thin films of Sm 2 O 3 (111) on Pt(111) has been studied using scanning tunneling microscopy(STM), low energy electron diffraction (LEED), and auger electron spectroscopy(AES). The films were grown by physical vapor deposition of samarium in a 5 X 10 7 Torr oxygen atmosphere. Continuous Sm 2 O 3 (111) f ilms were obtained by post growth annealing at 1030 K. Furthermore, heating the low coverage film in UHV causes it to partially reduce to form SmO(100). Re oxidation of the film reversed these changes along with improved morphology and ordering of the film

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9 CHAPTER 1 INTRODUCTION and electronics industries. Catalytic applications of interest include complete and partial oxidation reactions, oxidative elimination, hydrogenation and dehydrogenation rea ctions, coupling reactions and the selective reduction of NO, to name a few. Most rare earth oxides are thermally stable, as well as chemically active. REOs exhibit variable valence states along with high oxygen mobility within the oxide lattice. These pro perties allow the REOs to actively participate in surface redox reactions whereby lattice oxygen is exchanged with adsorbed reactants. REOs differ in their selectivity toward partial vs complete oxidation due to variations in oxygen mobility and the ease o f reduction/oxidation among t he REO series. Praseodymia and t erbia have the highest mobility within the REO series and can exist in multiple oxide phases, ranging from sesquioxide(Ln 2 O 3 ) to the dioxide(LnO 2 ) including mixed oxide phases such as Pr 6 O 11 and Tb 4 O 7 This ability to switch between multiple oxide phases facilitates oxygen exchange with adsorbates and promotes complete oxid ation of adsorbed reactants by c eri a, praseodymia and t erbia. On the other hand, REOs with lower oxygen mobility exist predomi nantly in the sesquioxide form and have a tendency towards partial oxidation of adsorbates. which provides a direct route for producing higher hydrocarbons from methane, and thus avoiding the sequential steps needed in indirect routes such as CH 4 reforming and Fischer Tropsch synthesis. One of th e principal products of OCM is e thylene which is used in products as diverse as food packaging, eyeglasses, cars, medical devices,

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10 lu bricants, engine coolants and liquid crystal displays(LCDs). The ability to convert methane to ethylene is highly attractive from an economic point of view because it has an estimated market of $160 billion/year. Sesquioxides of certain lanthanides such a s samaria (Sm 2 O 3 ), which do not form higher oxides, are effective in selectively promoting CH 4 coupling to ethane and ethylene, whereas ceria, praseodymia and terbia exhibit much lower selectivity for the OCM, and instead tend to completely oxidize methane to CO 2 and H 2 O. However, the activity of samaria and similar lanthanides is relatively low for the OCM. The factors which determine reaction selectivity of REOs are understood only to a limited extent, with most of the model surface science studies focusi ng on ceria. These contrasting catalytic properties within the REO series provide us substantial motivation for pursuing a detailed understanding of their chemical properties. Such an understanding would be crucial in the successful design of REO based cat alysts for use in a variety of applications. Here, we study the growth of Sm 2 O 3 films on Pt(111) surface. p latinum as substrate because of its thermal stability and high resistance towards oxidation. Sm 2 O 3 condenses in the bixbyite crystal str ucture. Bixbyite has a body centered cubic unit cell with 80 atoms and is of space group Ia 3 symmetry. Sm 2 O 3 has a lattice parameter a of 10.93 1 ,2 Sesquioxides like Dy 2 O 3 In 2 O 3 Pr 2 O 3 La 2 O 3 also exhibit this crystal structure. The bixbyite unit cell can be viewed to consist of 16 fluorite unit cells with a periodic arrangement of anion vacancies(one fourth of the anion sites are vacant). In the bulk, each Sm atom is six fold coordinated to oxygen atoms. All oxygen atoms have a tetrahedral coordination to four Sm neighbors. Therefore, similar to the fluorite structure the (111) planes are expected to be the surfaces with lowest

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11 energy. The surface unit cell of a bulk truncated Sm 2 O 3 (111) plane is hexagonal with a surface lattice parameter of 15.6 Sm 2 O 3 is an insula tor with a wide band gap of ~4.3 eV. 4,5 The studies presented here aim at creating a surface science model system that will enable studying surface properties of samaria. We show that thin samaria films adopt a cubic bixbyite structure and the films expose the low energy (111) face. The films exhibit a crystallographic relationship with the Pt(111) substrate. Using thin films supported on a metal enabled us to perform scanning tunneling microscopy (STM) and determine the surface structure of thin samaria films. Figure 1 1 .Rare Earth Consumption (REO) by Application. 3

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12 CHAPTER 2 CURRENT STATE OF KNOWLEDGE The rare earth elements oxidize with varyi ng strength. Under suitable conditions, all the rare earth elements form a sesquioxide The tendency to promote partial or total oxidation reactions is strongly influenced by the existing oxidation states and oxygen mobility: While ceria (where 4+ is the most stable oxidation state) is a good catalyst for total oxidation, samaria (where 3+ is the only stable oxidation state) seems to be the most effective REO catalyst for oxidat ive coupling of methane. D espite the high selectivity of samaria catalysts, the activity is too low for industrial applications. The first step in the OCM reaction is the activation of methane by the formation of a methyl radical. Oxides of cerium, praseodymium, terbium with high and multiple oxidation states actively promote the surface reaction of these CH 3 radicals and transform them finally into CO 2 On the other hand, CH 3 radicals have less affinity towards reac ting with the surface of samarium sesquioxide resulting in a largely gas phase reaction of these radicals to form coupling products. A couple of model REO thin f ilms on Pt(111) have been describe d in this chapter. Ultra Thin Ceria F ilm on Pt(111) CeO 2 ultrathin films were grown on the Pt(111) surface by reactive deposition of Ce using molecular or atomic oxygen as the oxidizing gas. High temperature treatments in O 2 produced epitaxial structures with a very good quality in terms of morphology, stoichiometry, and structure. The cerium oxide films have a very flat morphology with terraces several tens of nanometers wide. The stoichiometry of the films is mainly CeO 2 and the concentration of Ce 3+ io ns in the film can be reversibly increased by temperature treatments. The Ce 3+ concentration can be minimized by the use of atomic

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13 oxygen instead of molecular oxygen as oxidizing gas during the growth. The surface lattice parameter of the obtained ceria ov erlayers is smaller than the bulk one at all of the investigated thicknesses. The defects have been characterized using STM and were found to be more reactive than the terraces. 8 Figure 2 1. STM images of ceria film grown on Pt(111). A) 2ML ceri a sa mple as grown (2 V, 0.15 nA). B) 0.2ML ceria sample annealed in O 2 at 1040K(1V, 0.2 nA). C) Same as panel B (0.5 V, 0.2 nA). D) S ketch of the ceria/Pt system at low coverages with PtO 2 islands of different thicknesses either below the ceria isla nds or on the b are substrate. E ) 0.7ML ceria sample annealed in O 2 at 1040K measured at two different biases (left 0.3 V, 0.1 nA, right 1V, 0.1 nA). F) A tomically resolved images measured on the ceria islands o f the sample shown in panels B and C (0.8 V, 0.2 nA). T he different kinds of defects are evidenced oxygen vacancy clusters (triangle, zoo m in panel G ), surface oxygen va cancies (circle, zoom in panel H ), and subsurface oxygen vacancies (dashed circle, zoom in panel I ) from Luches et al 6 A B C D E F G H I

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14 Ultra T hin Y 2 O 3 (111) F ilms on Pt(111) B ulk like yttria films of sub nanometer thickness were grown on Pt(111). The films exhibit a Y 2 O 3 (111) 11 surface with a strict in plane orientation relationship with respect to the Pt substrate, thus forming a mono crystalline film. The surface structure of the yttria films was determined as the Y 2 O 3 (111) bulk truncation. LEED and STM images showed a large unit cell for the bixbyite structure. Furthermore, Y 2 O 3 (111) surface has been observed to be highly active for adsorption of hydroge n. Figure 2 2 STM images of Y 2 O 3 film grown on Pt(111). A ) Clean Pt(111) substr ate. The inset in A is the LEED (E=60 eV) pattern for the clean substrate. B ) ~1.5 ML yttria deposited on Pt(111) at room temperature. This film is subsequently anneal ed to C) 50 0 C, D) 600 C and E) 700 C. A 120 angle is drawn in E demonstrating the characteristic edge orientation. The line profil e along the indicated line in E is shown in F All STM images shown are 2525 nm 2 A ball and stick model of a cross sect ion through the bixbyite structure is shown in G to indicate the layered structure; red balls and light blue balls correspond to O and Y atoms, respectively from Tao et al 7 A B C G F E D

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15 CHAPTER 3 G OALS An essential aim of the project is the prepar ation of thin samari a films on a well defined metal substrate like Pt(111) The films will be grown by physical vapor deposition( PVD ) of the metal in an oxygen ambient. We will characterize the properties of films prepared in diffe rent oxidation states and the thermal stabili ty/phase transformations of the films using in situ analytical tools like STM, LEED, AES and TPD. D etailed characterization of the surfaces during thermal reduction/oxidation is important for determining atomistic processes governing phase transformations. focusing on the characterization of defect structures by STM, since they are expected to play an important role in the adsorbate interactions and the phase transfor mations. Apart from the pure films, we will also study how alkali doping affe cts the film p roperties like surface stru cture, thermal stability etc

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16 CHAPTER 4 EXPERIMENTAL M ETHODS The experiments were performed in a n u ltra high vacuum (UHV) chamber with a base pressure in the low 10 10 Torr range. The UHV system is equipped with a RHK scanning tunneling microscope (STM), OCI low energy electron diffraction (LEED) optics and Auger Spectrometer. The Pt(111) crystal was prepared by cycles of Ar + sputtering (1.5 keV) at 573 K followed by annealing at 1000 K in UHV until a well order ed Pt(111) pattern was observed in low energy electron diffraction (LEED) and impurities were below the detection limit of Auger electron spectroscopy (AES) In order to remove minor carbon contamination, cycles of annealing at 1030 K in 5 10 7 T orr of O 2 were also performed. After this procedure a sharp 1X1 pattern was observed for the Pt(111) substrate. STM images of clean Pt(111) showed terraces with step edges. The step heights measured were in the range of 1.80 2.00 against the reported value of 2. 30 pointing to an average error of ~17 %. 10 The Pt substrate was held at 600 K, with the samarium metal evaporated from an electron beam evaporator and with a background O 2 pressure of 5 10 7 torr. This partially formed oxide film was then annealed to 1030 K in the same O 2 atmosphere for 10 minutes. The rate of evaporation was determined to be approximately 0.3 ML/min for a flux current of 5 nA from AES. 9 thick samaria layer, i.e. the separation between two equivale nt (111) cleavage planes of Sm 2 O 3 All STM images were recorded i n constant current mode at room temperatu re with platinum iridium tips. The large scale STM images were acquired with bias voltages of 0.6 V for low coverage

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17 films and 1 2 V for thicker films with a tunneling current of 0.3 1 nA. Atomically resolved images were obtained at bias voltages of 0.6 V, 0.01 V and 1.34 V.

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18 CHAPTER 5 R ESULTS Three primary modes of thin film growth at a crystal surface are Volmer Weber(VW) growth, Frank van de r Merwe(FM) growth and Stranski Krastanov(SK) growth. Sm 2 O 3 thin films grow in SK mode on Pt(111). This is basically a layer plus island growth mechanism Initially, films of Sm 2 O 3 are formed followed by growth through nucleation and coalescence of Sm 2 O 3 islands. Figure 5 1 ( B ) ( D ) shows step profiles of macroscopic STM images ( A ) ( C ) The Pt step heights with Sm 2 O 3 present measure ~1.8 0 2 .00 and are consistent with those of the clean surface. Therefore, the error in our island height measurements shou ld range between 15 20 %. Figure 5 1 Pt step heights in the presence of Sm 2 O 3 A C are macroscopic STM images of 10 sec(~0.05 ML) and 2 min(~0.6 ML) samaria deposition on Pt(111) followed by annealing in O 2 B D are the line profile s along the indicated line s in A C respectively. E ) C ross section through the bixbyite structure 3.19 A B C D E 2 1.8

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19 For a (111) oriented Sm 2 O 3 film one may expect step heights of ~3.2 because of the separation between oxygen samarium oxygen trilayers in the bixbyite crystal s tructure, as is illustrated in figure 5 1 (E) Sm 2 O 3 forms a 1X1 structure at low coverage and with increasing coverage forms a 3X3 structure which seems to fade at high er coverages as seen from the LEED images. Also, with increasing cove rage the intensity of Sm spots is increasing while that of Pt spots is decreasing due to decreased electron scattering from Pt lattice. Thin Sm 2 O 3 films exhibit a crystallographic relationship with the Pt(111) substrate by exposing the low energy(111) surf ace. Figure 5 2 shows the LEED and STM images of the surface after deposition of ~0.6 monolayer(ML) of samaria on Pt(111) substrate followed by post annealing at 1030 K. The LEED pattern shows Samaria(111) 1X1 structure. Using the Pt(111) diffraction patte rn as a reference enables us to calculate the r atio of l attice constants as a /b =b/a=1.375. Using the lattice constant ratio, the lattice parameter of the samaria film is determined as 1.375*2.775 = 3.81 The average height of the island s observed is ~ 1 while the average depth of the holes is ~ 0.9 For samaria films with insufficient thickness to cov er the entire surface with a mono layer structure, areas of pure Pt or Pt covered only with a disordered samaria wetting layer remain exposed. The averag e diameter of the islands is ~3.3 nm Figure 5 3 shows the LEED and STM images of the surface after deposition of ~1.5 monolayer(ML) of samaria on Pt(111) substrate followed by post annealing at 1030 K. The average height of the islands measure ~1.2 Th e average diameter of the islands is ~4.1 nm

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20 Figure 5 4 shows the LEED and STM images of the surface after deposition of ~2.4 monolayer(ML) of samaria on Pt(111) substrate followed by post annealing at 1030 K. The extra spots around samarium in the LEED p attern are most visible at this film coverage. A preferential crystallographic orientation of the island edges is now observed. The edges of the islands reveal characteristic structure, typically enclosing angles of ~120. Islands measuring heights of 1.2 1.5 and 3 3 .5 are observed. Island heights of 1.2 1.5 shoul d be seen in the context of clean Pt surface having steps measuring 1.8 2 which would approximately add up to the oxygen samarium oxy gen trilayer separation height. Figu re 5 2 LEED and STM images of ~0.6 ML Sm 2 O 3 film grown on Pt(111). A ) LEED (E=58 eV) pattern for the 2 min(~0.6 ML) deposition of samaria followed by annealing in O 2 B, C are macroscop ic STM images of the surface. D, E are the line profiles along the ind icated lines in C The average diameter of the islands is ~6 nm. The atomic resolution of these islands reveal a imperfect hexagonal structure with voids. The average Sm Sm distance 1.2 1.5 B C D E A

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21 measured was ~3.2 which is in good agreement with 3.81 measured using LEED These images were scanned at a bias voltage of 0.6 V pro bing the unfilled states of Sm. Figure 5 3 LEED and STM images of ~1.5 ML Sm 2 O 3 film grown on Pt(111). A ) LEED (E=48 eV) pattern for the 5 min(~1.5 ML) deposition of samaria fol lowed by annealing in O 2 B, C are macroscop ic STM images of the surface. D ) L ine profile along the indicated line in C Figure 5 6 shows the LEED and STM images of the surface after deposition of ~4.2 monolayer(ML) of samaria on Pt(111) substrate followed by post annealing at 1030 K. The LEED patte rn show s fading of spots around samarium The average diameter of the islands is ~6.3 nm Atomic resolution of an island show a hexagonal s tructure consistent with the observation at a lower coverage We did not observe any large unit cell similar to that observed in the case of Y 2 O 3 on Pt(111). 7 Figure 5 7 shows the LEED and STM images of the surface after deposition of ~6 monolayer(ML) of samaria on Pt(111) substrate followed by post annealing at 1030 ~1.2 A B D C

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22 K. Most of the surface is covered with at least a monolayer of samaria and typical island heights measure ~3.5 Figure 5 4 LEED and STM images of ~2.4 ML Sm 2 O 3 film grown on Pt( 111). A) LEED (E=58 eV) pattern for the 8 min(~2.4 ML) deposition of samaria followed by annealing in O 2 B, C are m acroscop ic STM images of the surface. D) A tomic re solution of the box shown in C. E) L ine profile along the in dicated line in C Figure 5 5 Plots of average island diameter and lattice constant ratio wi th increasing coverage. ~2 A D E C B

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23 Figure 5 6 LEED and STM images of ~4.2 ML Sm 2 O 3 film grown on Pt(111). A) LEED (E=52 eV) pattern for the 14 min(~4.2 ML) deposition of samaria followed by annealing in O 2 B) M acrosc opic STM image of the surface. C ) A t omic reso lution of one of the islands. D) L ine profile along the indicated line in B Figure 5 7 LEED and STM images of ~6 ML Sm 2 O 3 film grown on Pt(111). A) LEED (E=56 eV) pattern for the 20 min(~6 ML) deposition of samaria followed by annealin g in O 2 B) M acrosco pic STM image of the surface. C) L ine profile along the indicated line in B UHV Treatment At a low coverage of Sm 2 O 3 heating the Sm 2 O 3 film in UHV for 30 min leads to the formation of a partially reduced new structure. The LEED patter n shows satellite spots with hexagonal symmetry around the Sm spots as seen in figure s 5 8(A) & (B), 5 9 (A ) This might be due to the formation of samaria platinum alloy Similar observation ~3.5 ~ 1.4 A B C D C B A

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24 has been reported with ceria and platinum. 6 ,11,12 STM images in figures 5 8(C) & 5 9 (B ) show th e surface cracking up with holes. The atomic resolution of one of these cracks at a bias voltage of 0.9 mV show a square lattice structure that can be attributed to SmO(100) as seen in figure 5 9 ( C ) The average Sm Sm distance measu red were 3.1 against an expected value of 3.55 Figure 5 8 LEED and STM images of ~1.5 ML Sm 2 O 3 film grown on Pt(111) follo wed by annealing in UHV. A B are LEED (E=64 eV & 39 eV) patterns for the 5 min(~1 .5 ML) deposition of samaria. C) M ac rosco pic STM image of the surface. D) A tomic resolution of one of the islands on the surface. There are instances of the co existence of the hexagonal and square domains as well as superimposed structures(Moi re pattern square+hexagonal). At a high coverage of Sm 2 O 3 we did not observe any reduced structure as seen in figure 5 10 One simple explanation would be that the Sm 2 O 3 layer is sufficiently thick enough so as to not D C B A

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25 expose the beneath SmO layer. One possible inference could be that the Pt Sm interfac e is responsible for this phenomenon. Figure 5 9 LEED and STM images of ~2.4 ML Sm 2 O 3 film grown on Pt(111) followed by annealing in UHV. A) LEED (E=58 eV) pattern for the 8 min(~2.4 ML) deposition of samaria. B) M acrosco pic STM image of the sur face. C) A tomic resolution o f one of the cracks shown in B Figure 5 10 LEED and STM images of ~ 6 ML Sm 2 O 3 film grown on Pt(111) followed by annealing in UHV. A, B are LEED (E=43 eV & 36 eV) patterns for the 20 min( ~6 ML) deposition of samaria. C) M acroscopic STM image of the surface. Reoxidation Re oxidation of the reduced surface reverses the change and goes back to the hexagonal structure of Sm 2 O 3 with changes in the morphology and ordering of the film as seen in figure 5 11 (B ) But the most no ticeable change is that the islands coalesce and form larger islands. Typical enclosing angles are ~120 o The atomic resolution of an island point towards a hexagonal structure as seen in fig ure 5 11 (C ) A B C A B C

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26 Figure 5 11 LEED and STM images of Re oxidi zed Sm 2 O 3 fi lm grown on Pt(111). A) LEED (E=58 eV) pattern for the 8 min(~2 .4 ML) deposition of samaria. B) M acroscopic STM im age of the surface. C) A tomic resolution of one of the islands on the surface. A B C

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27 CHAPTER 6 CONCLUSIONS AND FUTURE WORK W e have de m onstrated that thin samaria fi lms can be grown on Pt(111). The films exhibit a strict in plane orientation relationship with respect to the Pt substrate. Adsorption studies of CO can be conducted on Sm 2 O 3 films of different coverages as well as on the redu ced surface which will help us better u nderstand the nature of the holes STM probe could be used to further investigate the surface defects and vacancies along with their role in surface redox reactions including OCM reaction. The demonstration of the for mation of ordered thin films of samaria on a metal substrate will allow using this system to investigate the chemical properties of samaria by surface science techniques.

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28 LIST OF R EFERENCES (1 ) Adachi, G.; Imanaka, N. The Binary Rare Earth Oxides. Chem. R ev. 1998 98 1479 1514. (2 ) Villars, P.; Calvert, L. In Pearson's handbook of crystallographic data for intermetallic phases; American Society for Metals: 1985; Vol. 3, pp 4764 5000. ( 3 ) Kingsnorth, D. In In Rare Earths: Facing New Challenges in the New Decade; SME Annual Meeting; Society for Mining, Metallurgy, and Exploration (SME): 2010; ( 4 ) Dakhel, A. Dielectric and Optical Properties of Samarium Oxide Thin Films. J. Alloys Compounds 2004 365 233 239. ( 5 ) Constantinescu, C.; Ion, V.; Galca, A. C.; Dinescu, M. Morphological, Optical and Electrical Properties of Samarium Oxide Thin Films. Thin Solid Films 2012 520 6393 6397. (6 ) Luches, P.; Pagliuca, F.; Valeri, S. Morphology, Stoichiometry, and Interface Structure of CeO 2 Ultrathin Films on Pt(111). J. Phys. Chem. C 2011 115 10718 10726. (7 ) Tao, J.; Batzill, M. Ultrathin Y 2 O 3 (111) Films on Pt(111) Substrates. Surf. Sci. 2011 605 1826 1833. (8 ) Grinter, D. C.; Ithnin, R.; Pang, C. L.; Thornton, G. Defect Structure of Ultrathin Ceria Film s on Pt(111): Atomic Views from Scanning Tunnelling Microscopy. J. Phys. Chem. C 2010 114 17036 17041. (9 ) Dvorak, F.; Stetsovych, O.; Steger, M.; Cherradi, E.; Matolinova, I.; Tsud, N.; Skoda, M.; Skala, T.; Myslivecek, J.; Matolin, V. Adjusting Morphol ogy and Surface Reduction of CeO 2 (111) Thin Films on Cu(111). J. Phys. Chem. C 2011 115 7496 7503. (10 ) Zhang, L.; Kuhn, M.; Diebold, U.; Rodriguez, J. Thermal Stability of Ultrathin Cr Films on Pt(111). J Phys Chem B 1997 101 4588 4596. (11 ) Baddeley, C.; Stephenson, A.; Hardacre, C.; Tikhov, M.; Lambert, R. Structural and Electronic Properties of Ce Overlayers and Low Dimensional Pt Ce Alloys on Pt{111}. Phys. Rev. B 1997 56 12589 12598. (12 ) Essen, J. M.; Becker, C.; Wandelt, K. In In Pt x Ce 1 x Sur face Alloys on Pt(111): Structure and Adsorption; Conference ICSFS 14 ; e Journal of Surface Science and Nanotechnology: 2009; Vol. 7, pp 421 428.

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29 (13 ) Santos, D. M. F.; Saturnino, P. G.; Maccio, D.; Saccone, A.; Sequeira, C. A. C. Platinum Rare Earth Intermetallic Alloys as Anode Electrocatalysts for Borohydride Oxidation. Catal. Today 2011 170 134 140. (1 4 ) Eyring, L. In The binary rare earth oxides; Eyring, L., Gschneidner, K., Eds.; Handbook on the Physics and Chemistry of Rare Earths; Elsevier: 1979; Vol. 3, pp 337 399. ( 15 ) Petit, L.; Svane, A.; Szotek, Z.; Temmerman, W. M. Electronic Structure of Rare Earth Oxides. Rare Earth Oxide Thin Films: Gr owth Characterization and Applications 2007 106 331 343. (16 ) Unertl, W. In Physical Structure; Holloway, S., Richardson, N., Eds.; Handbook of Surface Science; Elsevier: 1996; Vol. 1, pp 271 360.

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30 BIOGRAPHICAL SKETCH Santosh Redd y Epuri was bor n in Hanamkonda, Andhra Pradesh, India to Sampath Reddy Epuri and Samatha Epuri. He graduated from FIITJEE Junior College, Hyderabad in 2007 and received hi degree in Chemical Engineering from the National Institute of Technology(NIT), Warang al in 2011. He enrolled in the m program at the University of Florida, Gainesville in the Fall of 2011. In January 2012, he